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narrator: given the earth's distance from the sun, its global-average temperature should be a chilling -18 degrees celsius. fortunately, gases in our atmosphere trap some of the sun's heat, creating a hospitable, average temperature of 15 degrees celsius. but the burning of fossil fuels is changing the composition of the atmosphere by increasing the concentrations of these heat-trapping gases. scientist pieter tans and a team of researchers are continuously keeping watch on this change -- one that could ultimately alter the environment for all living things. dr. tans: we use the atmosphere as a diagnostic to monitor that the earth is really doing what we hope it is doing.

narrator: but the atmosphere does not just trap heat, it moves it around the world, and together with the oceans shapes earth's climate and weather patterns. one powerful example of this is the tropical cyclone, more commonly known in the united states as the hurricane. dr. kerry emanuel has spent much of his career trying to understand these lethal storms and the larger role they might play in regulating climate. there's something rather unexpected that's turned up from research -- that hurricanes may have a profound effect on the climate. narrator: both of these studies offer us deeper insight into the complicated, ever-changing, active system that is the outermost layer of our planet -- the atmosphere. all around the world, scientists from

the national oceanic & atmospheric administration, commonly known as noaa, are monitoring trace gases in the atmosphere. one of their findings is that carbon dioxide is increasing due to the burning of fossil fuels on earth -- an increase that could have significant effects on the climate. for lead investigator pieter tans, the idea that our behavior could have any influence on the planet came as a surprise. dr. tans: i ran into a little book, and it was called "inadvertent climate modification." this was in 1972. my first reaction was, "that's nonsense. "i mean, how can we humans "influence the climate of this planet? "you know, we're just too small for that." but i started -- i leafed through it and i saw about infrared absorption by certain gases, and i thought, "okay, there may be something to that." so i bought the book. and i was convinced right then

that this was going to be an important problem. narrator: the problem, human-induced global warming, is caused by the heat-trapping characteristics of carbon dioxide and other so-called greenhouse gases -- the same characteristics that make the earth habitable. dr. tans: it's called the greenhouse effect. you can look at it like a blanket. so the earth receives energy from the sun, and, in fact, when we sit in the sun we feel that we're being heated. but it's not visible to the naked eye. it's infrared radiation. but the earth also emits that energy back into space. and when we put more of these absorbing greenhouse gases into the atmosphere, the infrared radiation will be prohibited from reaching space. it will be trapped in the atmosphere and emitted back to the surface. narrator: greenhouse gases in the atmosphere control the earth's temperature.

the greater the amount of greenhouse gases, the higher the temperature. less greenhouse gas means a lower temperature. carbon dioxide is cycled through the atmosphere through many natural processes such as photosynthesis, respiration and decay of vegetation, and sea surface gas exchange. this natural transfer, known as the carbon cycle, is in near balance. the amount being emitted to the atmosphere is close to the amount being absorbed. but the burning of fossil fuels is adding co2 faster than natural systems can respond. pieter tans is taking careful measurements of the atmospheric concentration of carbon dioxide to better understand the carbon cycle. dr. tans: when you study the carbon cycle, what matters is what happens to co2 exchange between the atmosphere and the oceans. what are terrestrial ecosystems doing? are they losing carbon? gaining carbon? why?

so we use the atmosphere as a diagnostic to get a handle on these processes to quantify where these processes take place and how large they are. we want to diagnose how the carbon cycle is developing, and we want to understand why. now, the first accurate measurements of carbon dioxide in the atmosphere were done in 1956 by david keeling. he was also the first to start continuous monitoring of carbon dioxide from a mountaintop in hawaii. the mauna loa volcano. narrator: noaa's global monitoring division has continued taking measurements at mauna loa for the past 30 years. but to enhance the accuracy of these measurements, they have expanded their data collection effort to many other remote locations across the globe. dr. tans: for many decades, we've had a global-flask-sampling network, whereby people send us air from specific locations

that are downwind from a large stretch of ocean water. or we get these samples from deserts or mountaintops. typically away from terrestrial vegetation. so we get air that is very clean and well mixed so that the weekly sample actually means something. it really indicates this is a concentration over a very large area. narrator: these samples are shipped to noaa's lab in boulder, colorado, where they are analyzed. in each sample, we measure carbon dioxide, methane, carbon monoxide, hydrogen, nitrous oxide, and sulfur hexafluoride. we'll take air from the bottle and send it to three different instruments. this is the one used to measure carbon dioxide. it's interesting -- the way we measure carbon dioxide is by absorption of infrared energy, which is the same principle as the greenhouse effect. so we're using the same property of the gas --

how it affects climate -- to actually measure it. then at night, we connect these suitcases that contain flask samples collected by aircraft. come back in the morning and they're all measured. combined, ground samples and aircraft -- up to 100 a day. narrator: from these hundreds of thousands of air samples, noaa has compiled a comprehensive database of climate-changing gas concentrations over a long period of time. their findings are consistent with those first discovered by david keeling. in the 1950s, keeling's first measurements showed co2 at the level of 315 parts per million. in 2005, noaa's measurements pegged co2 at 380 parts per million. beyond the steady rise in average co2 concentrations every year, noaa's data also showed that these levels fluctuate,

mainly due to the capture of carbon dioxide through photosynthesis. for example, during the northern hemisphere winter, carbon dioxide levels peak because so much vegetation is dormant at that time. on the other hand, during the northern hemisphere's summer growing season, co2 levels are at their lowest. this fluctuation can even be seen in a 24-hour period, reflecting the daytime uptake of carbon dioxide by plants when they are photosynthesizing and their release of carbon dioxide at night through respiration. the next step in noaa's diagnosis of the atmosphere is to take measurements close to where greenhouse gases are emitted or absorbed. carbon dioxide is emitted by natural processes, such as plant respiration and decay, as well as m-me processes, such as the burning of fossil fuel ese gas emitters are knowasources.

carbon dioxide is absorbed by terrestrial and oceanic plants through photosynthesis and also through sea surface gas exchange. these gas absorbers are known as sinks. if you talk about the budget of the greenhouse gases -- that is, to the atmosphere. that is the sum of the sources and the sinks. so if there's more sources than sinks, the concentration will go up. now, especially for north america, we do think that there's a significant sink -- significant absorption of carbon dioxide on this continent. right now we really can't tell where or why. so we need to do more specific measurements closer to where these sources and sinks are to try to untangle that. narrator: in north america, these measurements are being taken by noaa's tall-tower program led by noaa scientist arlyn andrews.

andrews is attempting to tease out north america's specific contributions to the global carbon cycle. we're trying to get measurements that are as close as possible to where all the action happens. so we want to be very close to the forests, close to the cities so that we can see the biggest signals. so what we do is we use a system like this, which measures carbon dioxide and carbon monoxide continuously. so 24 hours a day, we get a measurement every 5 minutes. narrator: john lee of the university of maine monitors one of the tall-tower sites. dr. lee: all we do is basically suck air all the way down the tower and send it to an analyzer that's down below. you want to be in the atmosphere, otherwise you'd have too much of the surface influence affecting your measurements. if we were right next to the surface, we'd only be measuring what's immediately below us

and upwind of us. by going up higher, we get all this well-mixed air from a much larger area. this is a pretty good example of northern forests that happen somewhere around this latitude all around the globe. there's kind of this green ring around the earth comprised of these northern forests. without forests like these, the levels of carbon dioxide would likely be higher, increasing the effect of global warming, because carbon dioxide is our major greenhouse gas. dr. andrews: north america does take a lot of co2. one reason that we've taken up a lot of co2, historically, is that when the eastern part of north america was settled, a lot of forests were cleared for agriculture. and those have been regrowing over the past 100, 150, or 200 years.

and so those forests have taken a lot of carbon dioxide out of the atmosphere, but they're nearing maturity, and so they probably won't be such a strong net sink for atmospheric carbon dioxide in the future. narrator: by increasing the number of tall towers, noaa scientists will enhance their ability to understand the rate that natural ecosystems are absorbing or releasing carbon dioxide. dr. andrews: we're really trying to develop a sampling network that will allow us to kind of keep our finger on the pulse of the biosphere continuously. so something that's sustainable and that we can use to monitor net carbon dioxide emissions for many years. narrator: taken together, the programs at noaa's global monitoring division have revealed not only how quickly greenhouse gases are rising in the atmosphere,

but are also providing a better understanding of sinks and sources in the cbon cycle. this information, in turn, will allow us to more accurately predict earth's future climate. dr. tans: when you're talking about increasing greenuse gases, you're talking clite change. see, there's one thing that is pretty certain. the biggest contributor to the rise in carbon dioxide is the emissions caused by burning coal and gas and oil. that's the biggest factor. it's bigger than natural processes that counteract these emissions a little bit. so if you can get an understanding of what the natural system -- what the earth itself is doing with this excess of gases that we've caused, we can actually make better prognoses of what future climate change might be like.

narrator: scientists are pondering if future climate change will affect hurricane activity on earth. some say hurricane katrina, which in 2005 devastated the gulf coast of the united states, is evidence that it already has. dr. kerry emanuel of the massachusetts institute of technology has been studying hurricanes for over 20 years. dr. emanuel: i mean, there was nothing meteorologically very exciting or interesting about katrina. what was unusual about it is it hit a very vulnerable place. but people have worried for some years about whether climate change affects hurricane activity. you know, everything from, "well, were there more or fewer hurricanes during the last ice age?" for example. "will there be more or fewer hurricanes if we have global warming?" there's a lot of argument about how this works.

narrator: while this argument continues today, historic records do point to a relationship between the recent warming of the oceans and the intensity of tropical cyclones. this graph shows a pronounced upward trend in the total energy generated by hurricanes over the past 50 years. this trend is well correlated with the upward trend in average tropical sea surface temperatures. but there is another question about the relationship between hurricanes and the climate that has received much less attention. could hurricanes serve a vital role in moderating earth's climate? dr. emanuel: very few people have asked the question, "do hurricanes play some central role in the climate?" we tend to think of them as freak storms that don't really affect the climate in an important way.

but there's something rather unexpected that's turned up from research -- that hurricanes may have a profound effect on the climate. narrator: for emanuel, it's questions like these that piqued an interest in hurricanes early in his career. dr. emanuel: often you think you understand something, and it's not until you have to teach it that you understand that you don't understand it. and this happened to me in the case of hurricanes. i was asked to teach a course in tropical meteorology. i thought i understood how hurricanes worked. i knew what the conventional explanation was. so i started to teach it, and i had one of those horrible experiences where you start talking about something and you realize it doesn't really make any sense. and that leads you down a path of trying to understand what does make sense. and that's what we call research. narrator: one of the most important tools emanuel uses in his work is computer modeling. dr. emanuel: most people believe that models are used principally to make forecasts, and of course they are.

but a lot of modeling is done not for the purpose of forecasting a particular phenomenon, but for understanding it. narrator: using models, emanuel can run simulated hurricanes hundreds of times a day and compare the results with data taken from real hurricanes. when differences are seen between the simulated hurricanes and the real ones, he can pinpoint the cause of those differences in the models and achieve not only better understanding of real hurricanes, but also create more accurate models for future research and forecasts. recently, one of those differences arose which may show hurricanes serving a function never before considered. in order for a tropical cyclone to reach the greatest intensity -- category five, like katrina did in 2005 -- it must have an unusually abundant supply of its raw fuel -- warm seawater. but not all real hurricanes reach this maximum intensity.

in fact, they rarely do. dr. emanuel: it's a speed limit. no storm ever exceeds this limit, and a very small percentage of them actually come right up to the limit. most of them fall far short. now, the computer models behaved in some ways very differently from real hurricanes in that they always spun up to their speed limit. narrator: essential to answering why the hurricanes in the models were behaving differently from real hurricanes is an understanding of how real hurricanes work. dr. emanuel: if you look at a photograph of a hurricane from space, what you see, of course, is this beautiful, white spiral mass of clouds. it's a little bit hard to tell, but those clouds extend from very close to the ocean surface up about 10 miles high. and the ocean surface is at a pretty high temperature -- 25 to 30 degrees centigrade. but the tops of the clouds in a hurricane represent the temperature that you find

in the upper part of the atmosphere. it's very, very cold. it's around -80 degrees centigrade. so the hurricane's operating across a rather spectacular temperature gradient. narrator: this temperature gradient is just one of the many reasons that hurricanes grow into such powerful storms. the warm ocean water evaporates into the cool air above it, taking heat energy out of the ocean and storing it in the form of water vapor. when that water condenses into clouds, this energy is transferred to the air, warming it and increasing wind speeds even further. dr. emanuel: once this process gets under way, the feedback that allows the hurricane to grow is one between wind and evaporation. the stronger the hurricane winds, the more evaporation. the more evaporation, the more heat goes into the hurricane. the more heat goes into the hurricane, the stronger it gets. and that would just go on forever,

but there's one other thing that's stopping it from doing that -- it's friction. narrator: friction is caused by the same thing that's aiding evaporation -- sea spray. dr. emanuel: sea spray is fundamental to a hurricane. when a spray droplet goes up and partially evaporates, it turns out that process transfers an enormous amount of heat to the air. and so sea spray is a very efficient heat-transfer mechanism. now, those spray drops are also slowing the atmosphere down. they're exerting a drag on the winds. the physics of that is very complicated. very complicated. and we don't understand it. and we're trying to understand those physics. we're trying to carefully measure the friction and the heat transfer of the spray. so we flew airplanes into hurricanes over the atlantic to make measurements to help us understand how sea spray affects this heat-transfer and momentum-transfer process.

and when we start to better understand this process, we can, with any luck, make better forecasts of how hurricanes will evolve. narrator: but overflights are limited in the data they can provide, so emanuel also studies hurricane machines to get a closeup view of what may be happening when the atmosphere and the ocean interact. dr. emanuel: so we have built a laboratory apparatus in which we can simulate these very fantastic conditions -- very high wind speeds, air blowing across water, the air is filled with spray. when the winds start to blow more than about 80 or 90 miles per hour, so much spray is lofted into the air that eventually it becomes hard to even talk about the surface of the ocean. and i'm not speaking metaphorically. you just go from water, which is filled with bubbles, to air, which is filled with spray gradually. and there's no longer anything you can call the surface of the ocean. so it becomes a real, real chaos in the core of a hurricane.

narrator: the interface between ocean and atmosphere is a vital clue to why all of the simulated hurricanes in emanuel's models were reaching maximum intensity, compared to real hurricanes, which rarely do. it also may show hurricanes playing a significant role in controlling the climate. dr. emanuel: in the idealized models, we hold the ocean temperature fixed. we don't let it change. it's just whatever it is. but a real hurricane profoundly changes the temperature of the seawater. not because it's taking heat out of the ocean -- it is doing that, and that does cool the water, but it's maybe a tenth of a degree or so. it's not really noticeable because it's such a huge heat reservoir. what the real hurricanes do is they churn up the ocean. you don't have to go very far down in the tropical ocean before you find very cold water.

it's only hot right within the first 100 feet or so of the surface. the hurricanes come along and they mix this cold water up to the surface. and you can look at a satellite picture to see these really cold wakes that are left behind by hurricanes. and so the hurricane is cooling off the ocean temperature. narrator: this ocean cooling offers some explanation as to why real hurricanes do not always grow to their maximum potential intensity. when this effect is incorporated into the computer models, the simulated hurricanes act much more like real storms. this finding implies that a hurricane's relationship with the atmosphere and the ocean may be much more dynamic than previously thought, leading to further implications for how the atmosphere and the oceans moderate earth's climate. dr. emanuel: the reason that the climate isn't more extreme than it is is because both the atmosphere and the ocean take heat from the tropics and bodily move it to the poles.

so we have warm air flowing from the tropics toward the poles. and we have cold air flowing from the poles to the tropics. the ocean does the same kind of thing. we have warm gulf stream waters flowing poleward. and underneath them, we have very cold water moving southwards. and if you change those transports somehow, you've changed the climate. narrator: emanuel's research is showing that hurricanes help the oceans to play this heat-transporting role. dr. emanuel: it was actually proved about 100 years ago that the only way you could make the ocean transport a lot of heat from the tropics to the poles is if you could turbulently mix hot water in the tropics down into the deep tropical ocean. that's what you need to do. and oceanographers argued for years about what's doing this mixing. we've done some calculations that suggest that global tropical cyclone activity --

"tropical cyclone" is a generic name for a hurricane -- is what's doing this mixing. narrator: putting this theory to the test, emanuel has built hurricane models where the hurricanes control the heat flux in the oceans. these models may provide an explanation for the climate on earth 50 million years ago -- the time known as the eocene heat wave, when the temperatures in the tropics were about the same as they are today, but the poles were much warmer. this is when there were crocodiles wandering around greenland and alligators in london and places like that. and this is a great mystery. but we think it was a very stormy climate and there were a lot of hurricanes and that this was driving a fantastic poleward heat flux in the ocean, which was responsible both for keeping the tropics relatively cool and for keeping the high latitudes warm. narrator: the idea that hurricanes are in some way

driving the earth's climate complicates our understanding of how the climate system works, presenting new challenges as we try to predict future climate and hurricane activity. dr. emanuel: now, if this is true, it means that we have to completely rethink our understanding of how the climate works in general. because in the big, huge computer models that are used to simulate climate, this mixing is just specified. it's constant. it doesn't change with time or climate. it's specified out of ignorance. we don't know what else to do. if it's hurricanes that are driving this mixing, we have a different problem altogether. we have a different system dynamic, because the hurricanes themselves are functions of the climate. so this is a new branch of research that now ties hurricanes into the whole climate system in a way that might prove to be very interesting.

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